One-way torque transmitting mechanisms have been employed in many automatic shifting transmissions to accommodate the ratio interchange in the transmission. The one-way torque transmitting mechanism is provided to either transmit torque from the engine to a gear member or transmit torque from the gear member to ground. As is well-known, the one way torque transmitting mechanism will release the controlled gear member upon a reversal of torque that occurs during the ratio interchange. This permits a smooth transition between ratios. The one-way mechanisms are mechanical devices that require space in the transmission and also add weight to the transmission.
To eliminate the use of one-way torque transmitting mechanisms, some transmission control systems have incorporated electrohydraulic control systems with "clutch to clutch" shift technology. The control systems have utilized two strategies, open loop control and closed loop control. During open loop control, the oncoming friction torque transmitting mechanism (clutch or brake) is filled with fluid and the pressure is ramped up to the inertial pressure required during the shift. The release timing of the pressure in the offgoing friction torque transmitting mechanism is based on an estimation of the oncoming torque transmitting mechanism fill time. The fill time of the oncoming torque transmitting mechanism varies due to many design and assembly factors such that the release of the offgoing torque transmitting mechanism can be early, causing a flare, or late, causing a tie-up. Some control algorithms have been developed to detect the oncoming clutch fill using an input or output speed signal. However, these have not proved reliable for practical use.
During closed loop control, the offgoing torque transmitting mechanism capacity is reduced to its critical point by generating a predetermined slip speed in the offgoing torque transmitting mechanism. The oncoming torque transmitting mechanism is filled and ramped up to the inertial pressure. As the oncoming torque transmitting mechanism gains capacity, the input speed will drop. As the input speed drop is detected by the microprocessor, the offgoing torque transmitting mechanism capacity is reduced to zero. In the closed loop control, there is a controlled engine flare at the beginning of the interchange causing an output torque dip. Also since the offgoing torque transmitting mechanism is not released until the input speed drop is detected, a tie-up is present during the ratio interchange.